1. Field of the Invention
The present invention relates to a pneumatic tire provided with a tread pattern having a transverse groove formed between a plurality of blocks, and is particularly useful for a studless tire.
2. Description of the Related Art
In general, various types of tread patterns are formed in the pneumatic tire for preventing a slip with respect to a road surface during the driving, however, since a performance of the tire having the normal tread pattern is inferior on an iced or snowed road, a special tire is used for driving on the frozen road.
As the tire mentioned above, a studless tire using no metal stud becomes popular at present. The studless tire has a comparatively soft rubber tread, is provided with a lot of blocks on a surface thereof in a projecting manner, and is generally structured such that a plurality of sipes (narrow grooves) is formed in a tire width direction with respect to each of the blocks. In accordance with the studless tire, it is possible to improve a friction force applied to a road surface, by sucking up a water screen on the frozen road by the sipes.
On the other hand, with respect to the iced and snowed road, since the groove between the blocks of the studless tire is deep and wide, the hard packed snow is filled in the groove, and a certain degree of driving force and braking force can be generated via the snow. However, in the case that an air temperature rises and the iced and snowed road is in a so-called sherbet state, there is a lot of cases that the normal studless tire can not generate the sufficient driving force and braking force.
For the purpose of solving the problem mentioned above, the Japanese Unexamined Patent Publication No. 7-81318 discloses a studless tire provided with a lot of long anti-slip holes corresponding to a long quadrangular recess in which both side surfaces are formed as a slope surface and provided with a hill in a bottom thereof, a lot of circular anti-slip holes corresponding to a bowl-shaped recess, provided with a water reservoir in a center and forming an annular step portion in the middle thereof, a lot of rectangular anti-slip holes corresponding to a quadrangular recess in which both side surfaces are formed as a slope surface, provided with a water reservoir in a center portion and forming a step portion in the middle of the slope surface, a lot of diagonal anti-slip holes corresponding to a quadrangular recess in which both side surfaces are formed as a slope surface, provided with a water reservoir in a center portion and formed so as to be sloped with respect to a width direction of the tire, a lot of anti-slip chips in which a fibrous chip made of a good water absorbing material is buried within a depressed portion having an appropriate depth and an appropriate shape, and a lot of drain grooves coupling the portions mentioned above.
However, in the studless tire, since groove widths of the drain groove coupling the respective portions and the drain grooves connected to the tread end are narrow, it can not be said that a draining effect is sufficient with respect to the road surface on the snow in the sherbet state, a state close to a hydroplaning tends to be generated, and a sufficient driving performance is hard to be obtained.
Further, for the purpose of increasing the driving performance on the road surface of the snow in the sherbet state, there can be considered a method of increasing the drawing performance by enlarging the width of the transverse grooves existing between the blocks in such a manner as to be wider in the tread end side. However, in this case, since the rigidity of the block is partly lowered in the portion having the wide groove width due to the different width of the transverse groove, a biased wear such as a toe-and-heel wear or the like tends to be generated.